1. Field of the Invention
The present invention generally relates to the field of software encryption, and specifically relates to protecting computer programs distributed using a broadcast media.
2. Brief Description of Background Art
Broadcast distribution of software, particularly software designed for personal computers, has introduced several problems of access and distribution control. For purposes of this discussion, the term "distribution" means distributing a large number of different software titles to a large number of potential customers without the ability to deliver different copies to each user. In addition to the usual mode of transmitting software on floppy diskettes, software may readily be transmitted via radio frequency public broadcast or computer network, each medium having the same problem of controlling access to the software thus transmitted.
The widespread use of CD-ROM with personal computers has provided an opportunity for a new model of retail software distribution. These disks have high capacity, typically equaling that of 400 floppy diskettes. The manufacturing cost of CD media is approximately equal to that of a single floppy diskette, thus allowing for low cost distribution of numerous different software products on a single CD.
However, CD-ROM distribution creates a problem of how to allow customers to pay only for those products that they wish to purchase, while preventing access to those products that physically are on the CD but have not been purchased. Encryption techniques offer a potential solution.
Encryption technology is a field that has been applied to software for decades. The problem addressed by most encryption systems is restriction of the ability to make copies of the subject software. Under such a scheme, the software is encrypted such that a copy of itself will not properly operate or the encryption in some way interferes with the proper functioning of a copy. One limitation of such a system is that an authorized user may desire to make back-up copies of the software, load it onto a computer internal memory, or otherwise make legitimate copies of the purchased software.
Some encryption systems, such as that disclosed in Thomas U.S. Pat. No. 4,446,519, integrate customized hardware that cooperates with the software. In that reference, an electronic security device (ESD) is attached to the computer, a software program provides a value that is sent to the device and processed, and the software checks for an expected response. Typically, the ESD is shipped with the software. Such a system only is directed to solving the problem of unauthorized copying of the software, not unauthorized access of distributed copies of the software.
In other systems, such as that disclosed in Matyas U.S. Pat. No. 4,757,534, each software medium containing the same program is different. That is, each floppy diskette containing the software has a unique diskette serial number. A password is given to the user to access the encrypted software. However, the password of Matyas is keyed to the unique medium, not to the software program itself. In addition, the Matyas system loads the decrypted data into a secure RAM. This addresses the problem of preventing unauthorized copying of the decrypted program. Such a scheme is not feasible in a broadcast distribution system in which it is cost-prohibitive if not impossible to assign a unique identification to each medium or where the software is broadcast over a network.
Thus, there remains a need for a method and system that allow the secure distribution of software on a broadcast medium, such as on CD-ROM or over a network. Such a method and system are needed to allow distribution of one or several computer programs, while allowing the publisher of the software to limit access to selected ones of the programs using the existing base of personal computers and personal computer designs.